The U.S. military has a continuing interest in wheeled armored vehicles. A comparison of wheeled and tracked vehicles reveals a major difference in silhouettes. As can be readily appreciated, a low silhouette offers many advantages in combat. The major contributing factors to this difference in silhouette are the automotive-type drive train and steering mechanisms used in conventional wheeled armored vehicles. Hydraulic and electrical drive systems potentially allow configurations competitive with silhouettes of tracked vehicles.
To achieve this advantage of a hydraulic or electrical drive system, the wheel drive motor should be located as near as possible to the wheels. In the case of hydraulic drive systems, this requires much piping, hydraulic rotating joints, and complex road arms. Steering wheels become a very difficult hydraulic problem. A speed range of 3 to 60 mph is difficult to achieve when 60% grade torque is required at the low end and highway conditions at the upper end. The components become very large and efficiency is low. Weight of pumps, fluids, control valves, piping and cooling also appears to be excessive. Controls for a 6.times.6 hydraulic system are quite involved. Cold weather (-45.degree. F.) start up of a hydraulic system is time consuming. Reaction time to full power from idle is not fast. Momentary overloads, as might be required for evasive action, are not available with hydraulics because of pressure limitations. Consequently, there appears to be little advantage in pursuing the hydraulics drive train approach for a modern combat vehicle.
The state of the art in electric motors and their controls reveals a maturing technology in speed and torque control. There are a wide variety of types of electric drive motors, each with its own advantages and disadvantages. One of the most common types of drive motors is a stepper motor. This motor provides open loop position and velocity control. They are relatively low in cost and they interface easily with electronic drive circuits. Recent developments in control systems have permitted each stepper motor "step" to be divided into many incremental microsteps. As many as 10,000 or more microsteps per revolution can be obtained. Motor magnetic stiffness, however, is lower at these microstepping positions. Typically, stepper motors are run in an open loop configuration. In this mode they are underdamped systems and are prone to vibration, which can be damped either mechanically or through application of closed loop control algorithms. Power-to-weight ratios are lower for stepper motors than for other types of electric motors.
The permanent-magnet, direct-current, brush-commutated motor is widely available and comes in many different types and configurations. The lowest-cost permanent magnet motors are the ceramic (ferrite) magnet motors. Motors with alnico magnets have a higher energy product and produce higher motor constants than equivalent sized motors with ceramic magnets. (Motor constant is defined as torque produced divided by the square root of power consumed.) Rare-earth (samarium-cobalt) motors have the highest energy product magnets, and, in general, produce the largest peak torques because they can accept large currents without demagnetizing. However, these larger currents cause increased brush wear and more rapid motor heating.
Another subset of DC permanent-magnet brush motors are ironless rotor motors. Typicallly, these motors have rotors made of copper conductors enclosed in epoxy glass cup or disk rotor structures. The advantages of these motors include low inertia and negligible inductance, which reduces arcing, extends brush life, and results in short electrical and mechanical time constants. Because these motors have no iron in the rotor they have very little residual magnetism and consequently very low cogging torques. Disk-type motors have several advantages. They have short overall lengths, and because their rotors have many commutation segments they produce a smooth output with low torque ripple. A disadvantage of ironless armature motors is that they have a low thermal capacity due to low mass and limited thermal paths to their case. As a result, they have rigid duty cycle limitations or require forced-air cooling when driven at high-torque levels.
The weakest links in most motor designs are the bearings and brushes. Brushless DC motors, also classified as synchronous AC motors, have been developed. They substitute magnetic and optical switches and sensors nd electronic switching circuitry for the graphite brushes and copper bar commutators, thus eliminating the friction, sparking, and wear of commutating parts. Brushless DC motors generally have good performance at low cost because of the decreased complexity of the motor. However, the controllers for these motors are generally more expensive because they must include all of the switching circuitry. The cost, reliability and flexibility of such controllers, however, have also improved due to such features as improved semiconductor technology and the use of microprocessor control technology.
Brushless DC motors also have increased reliability and improved thermal capacity. This improved thermal capacity occurs because in brushless motors the rotor is a passive magnet and the wire windings are in the stator, giving them good thermal conductivity to the motor case.
The U.S. Pat. No. to Fengler 4,211,930 discloses a constant-speed, continuously-running, low-power diesel engine or turbine which drives a fixed-frequency, two-pase alternator, the output from which, for direct drive, flows to the stator pole piece windings of four independently-rotating stepping motors operating synchronously with the DC alternator. Each motor also includes a rotor having a plurality of circumferentially spaced, rare-earth magnets of alternating polarity. Each stepping motor is connected to its respective traction wheel of a motor vehicle, thereby propelled at a limited maximum speed sufficient to overcome normal wind resistance over a level road. The motors are connected to a suspension arm assembly and swivel around a hinge line at the center of the vehicle. In starting, during acceleration, and for propulsion at higher speeds, direct current from a storage battery is caused to pulsate and is added to the current from the alternator to the stepping motors. A solid state control circuit selectively controls the frequency of a variable frequency generator electrically connected to the pulse-responsive electrical power system to vary the frequency of the current supplied to the stepping motors and thus vary the vehicle speed. The frequency and phase of the stator currents are controlled through SCR's or thyristors to maintain the magnetic field and the maximum torque condition, independent of rotor speed or supply frequency. During idling, the alternating current from the alternator is rectified and recharges the battery. During braking, the consequent driving of the stepping motors causes them to generate alternating current which is rectified and returned to the battery. By varying the frequencies of the current delivered to the right side motors as compared with those delivered to the left side motors and vice versa, in response to the turning of the steering wheel in rounding a curve in the road, a differential action is obtained.
The U.S. Pat. No. to Ehrenberg 4,089,384 discloses a vehicle including a turbine-drive electric generator, and a plurality of independent motor-wheel systems. The AC voltage from the generator is rectified and the resulting DC voltage is supplied to the serially connected DC motors.
The U.S. Pat. No. to Kassekert et al 3,915,251 discloses an electric vehicle drive having a DC drive motor with shunt field control. A DC power supply is connected to the drive motor. Drive pulleys and belts transfer drive torque from the motor to the drive wheels.
The U.S. Pat. No. to Etienne 4,187,436 discloses an electric hybrid vehicle including driving wheels driven by an electric driving motor supplied with current by a battery. The battery is charged by an alternator which, in turn, is driven by an engine. The excitation winding of the alternator is controlled by a circuit which, in turn, is controlled by a circuit which monitors the state of the battery. Logic circuitry is also provided for controlling the engine.